In vivo the mammary epithelium rests upon a basement membrane containing type IV collagen. This collagen substratum is made primarily by a myoepithelial-like cell and appears to be essential for the growth and survival of the epithelial cell population since the epithelial cells degenerate and die in vivo and in vitro if collagen deposition is prevented following a proliferative stimulus. In vitro studies suggest that mammary growth control may be regulated by the action of basement membrane deposition by the myoepithelial cells. Involution is apparently triggered by the breakdown of preformed basement membrane catalyzed by a specific collagenase produced by the epithelium upon withdrawal of hormonal support. Some primary tumors appear to retain a requirement for basement membrane collagen synthesis since they regress when the host is administered cis-hydroxyproline to block collagen deposition.